Protection of apparatus against acids



Patented Aug. 9, 1932 NICOLAS BENOIT GRILLET, F NEUILLY-SUR-SEINE, FRANCE, ASSIGNOR, BY ASSIGNMENTS, TO E. I. DU FONT DE NEMOURSAND COMPANY, OF WILMINGTON,

DELAWARE, A CORPDKAI-IQN OF DELAWARE PROTECTION 0 rr Ai-vs ee I sr ACIDS No Drawing. Application filed November 17, 1927, Serial No. 234,046, and in Germany February 4, 1327 Certain carboxylic organic acids such as, for instance, formic, acetic, propionic, bu

' tyric, lactic, naphthenic acids and higher fatty acids, attack more or less strongly all the metals which are in current use in industrialpractice. On this account considerable difliculty is experienced in the purification and the concentration of these acids in carrying out the reactions in Whichthey take a part and also in their transportation and storage.

Copper and its alloys are the metals commonly used in the construction of the apparatus which contain'such acids, because they are more resistant to corrosion at various concentrations, but the corrosion of these metals is nevertheless conslderable; acetic acid, for instance, attacks them strongly enough to necessitate continual repairs and a periodical replacementof worn-out apparatusr Concerning acetic acid, the influence of air on the corrosion of metals has been Wellknown. Fleck (Centralblatt 1883, p. 1626) points out that acetic acid attacks metals,

and particularly copper,at lov temperature in the absence of air, and more strongly in the presence of air. It is also known that,

. for protecting copper or its alloys against the action of acetic acid.

In industrial operations the corrosion of copper or its alloys, even if the operations are carried out in the absence of air, is ofttimes very substantial.

. I have. found that this corrosion is due to the presence in these commercial products "of oxygen in the form of oxidizing combinations capable of becoming reduced in the plant and corroding it even in the absence of molecular oxygen. Among these oxidizing combinations is cupric acetatewhich is often found in industrial acetic acid, particularly in residual acids. 7

I have found that, if the operation is carried out at boiling temperature'in the presence of cupric acetate and in the absence of molecular oxygen, corrosion of the copper takes place to correspond to the solution of approximately one atom of metal for each atom of copper initially dissolved in the cupric state. It appears that the cupric salt gives oii a portion of its oxygen. Acuprous salt is thus formed and the corrosion ceases. If the copper is initially dissolved in the state of a 'cuprous salt, no corrosion phenomena takes place.

The process of protecting copper and its alloys against corrosion by carboxylic organic acids, particularly by acetic acid and its homologues, which is the object of the present invention, consists essentially in carrying out industrial operations in which these acids are used With a complete absence of oxygen in the molecular form andinthe' state of oxidizing'combinations capable of giving up oxygen under operating conditions. In order to realize this it s necessary to: i

1) Eliminate from all the materials entering in the reaction, oxygen chemically 2.0m. binedbut capable of oxidizing the metals "in the presence ofan acid. l

(2) Totally exclude all molecular oxygen inside the apparatus hether in gaseous form or in a state of solution in the materials used in the operation.

In order to fulfill the lirsircondition the oxidizing combinationslmay be preliminarily destroyed by suitable means or these oxidizing combinations may be transformed by re duction into products which contain less oxygen and which do not attack the apparatus in the absence of molecular oxygen. For example,if cupric salt is present in the product to be treated, the copper maybe preci )itated by electrolysis or by any other method of precipitation such as'displacement by iron .or any other suitable metal. The product may also be treated by suitable reducing reagents such as hypophosphorous, sulphurous, hydrosulphurous acidsor their salts, aldehydes, ketones or very oxidizable polyphenols.

In order to' fulfill the second conditionit is necessary to carry out the operationwith the exclusion of air and to remove the molecular oxygen from the acid and the reagents employed. For example, the free surface may be covered with a layer of suitable oil or the operation may be carried out in contact with an atmosphere which is inert, that is, free from molecular oxygen. The pressure of this atmosphere is advantageously greater than atmospheric pressure in order to avoid any accidental admission of air. I

For removing the molecularoxygen contained in the reacting bodies the usual methods may be used. For example, the liquid may be boiled before being used; a stream of gas free from molecular oxygen can be bubbled through the liquids. In these cases it Wouldbe advantageous touse the gas used as inert atmosphere. This process is quite efiicient. For example, to free acetic acid from dissolvedoxygen it is sufiicient to cause approximately three cubic meters or even less of an inert gas to bubble through a cubic meter of acid. The solids which may take part in the operation may be'similarly freed from occluded oxygen by the passage of a stream v.ofinert gas, The oxygen dissolved in the liquids may also be chemically fixed by treating them with reducing agents out of contact withair.

This last method of treatment was very practical, because the reducing agents used at the same time reduce the oxidizing products which are present in the material treated. It is sometimes noticed that these impure liquids are deoxygenated more rapidly, the oxidizing impurity acting asa catalyst in the deoxygenation. This is true for cupric salts for instance. It is alsopossible to obtain the desired results with verysmall quantities of suitable reducing compound. Thus, sulphurous acid, either free or in the form of salts, gives excellent results.

' It is very important in order to avoid the corrosion of the apparatus toexclude oxygen in all the operations taking place in connection with the acid used, even if the oxygen is small in quantity, such as, the operations of emptying and cleaning the apparatus. In opening the apparatus for repairs for example, air will be admitted only after all the acidity has been eliminated, for example, washing with water deprived of oxygen. Unless this precaution is taken, substantial corrosion will take place which will make it necessary to soon open the apparatus for new repairs and this opening would entail new corrosion and the repeated process would rapidly put the installation out of use.

My process .of protecting copper and its alloys is applicable to all physical or chemical operations employing organic carboxylic acids, particularly acetic acid or its homologuesz. distillation, extraction, concentration, transportation, acylation and various reactions in which they may play a part as reagents or as solvents or in any other way.

The examples of some of the methods for carrying outmy invention are as follows:

Emample I .-Diluted recovered acetic acid is concentrated by continuous extraction with ether, according to the principle described in the German Patent No. 28,06 of Gohring (extraction, separation of acid ether Before the extraction, the, acetic acid passes into a column filled with iron turnings'. By the action of the iron, the copper present in the form of cupric acetate is displaced and the dissolved oxygen is eliminated with solution of a certain quantity of iron. The acid contained in the column is sheltered from theair by'a layer of oil on'its upper surface. The copperdeposits are withdrawn at the bottom of the column.

The ether employedis also preliminarily freed from any trace of. dissolved oxygen. The apparatus is connectedto a gasometer filled with a gas not containing molecular oxygen, such as, for instance, gasogen gas free from its ultimate traces of molecular oxygen. The whole apparatus is of copper.

At the moment when the plant is started,

all the air in the apparatus is first replaced by gas from the gasometer. The products,

free from oxygen as mentioned above, are

apparatus, all trace of acidity is first eliminated by replacing the acid contained in the apparatus by deoxygejnated water stored,

sheltered from air under a layer of oil and washing the apparatus therewith.

A similar installation, working without the precautions indicated above, is substantially corroded and yields a concentrated acid containing notable quantities of copper salts. Thecorrosion entails such frequent interruptions for thepurpose of repairs that the working is rendered commercially impossible. i

Example 'I LThe rectification of acetic acid in copper apparatus is now carried on with considerable corrosion of the plant which entails very frequent repairs and rapid wear. This examplerelates to the industrial application of my invention to this operation.

The air contained in the'rectification apparatus is replaced by 'a gas free from molecular oxygen. The raw dilute acid con.

.tained in a tank is sheltered from air by a layer of oil. It is treated with a suitable quantity of sodium'bisulphite and then it is run into the rectification apparatus, taking careto prevent the passage ofoil.

The distilling apparatus carries a fractioning column at the top; the product of the disto the known processes.

tillati on is condensed and collected according The whole is maintained, during the rectification, in communication with a gasometer containing a gas free i from molecular oxygen at a pressure slightparticular method of operation as illustrative of my invention, I do not desire to limit myself to the same as various changes and modifications may be made, all within the scope of the appended claims.

.What I claim and desire to secure by Letters Patent is:

1. A process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein which comprises freeing the apparatus before use from substances capable of favoring the corrosion by saidacids of said apparatus, removing from the materials, prior to the introduction thereof in the apparatus, all oxygen including oxygen in chemical combination capable of favoring corrosion of said apparatus and protecting the materials in said apparatus from contact with air.

2. A process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein which comprises freeing the apparatus before use from substances capable of favoring the corrosion by said acids of said apparatus, removing from the materials, prior to the introduction thereof in the apparatus, all oxygen including oxygen in chemical combination capable of favoring corrosion of said apparatus and protecting the materials in said apparatus from contact with air with an inert gas.

- 3. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein, the steps which comprise treating the materials prior to the introduction thereof in the apparatus with a reducing agent to remove the molecular oxygen and maintaining the materials within the apparatus out of contact with oxidizing substances.

4. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein, the steps which comprise'boiling the acids prior to the introduction thereof in the apparatus to remove the molecular oxygen present therein and maintaining the materials within the apparatus out of contact with oxidizing substances.

5. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein, the steps which comprise passing an inert gas through the acids prior to the introduction thereof in the apparatus to remove the molecular oxygen present and maintaining the materials within the apparatus out of contact with oxidizing substances.

6. In a process of protecting metallic apparatus constructed of metal, thebase of which is copper, against corrosion by car-V boxylic acids introduced therein, the steps which comprise treating the acids prior to the introduction thereof with a reducing agent to eliminate the oxygen present in oxidizing combinations and maintaining the materials within the apparatus outof contact with oxidizing substances.

7. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein, the steps which comprise treating the acids prior to the introduction thereof in the apparatus with a reducing agentto simultaneously destroy the oxygen present in the molecular and oxidizing combination and maintaining the materials in the apparatus out of contact with oxidizing substances. V

8. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein, the steps whichcomprise treating the acids prior to the introduction thereof in the apparatus with sulphurous acid and maintaining the materials in the apparatus out of contact with oxidizing substances.

9. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by carboxylic acids introduced therein, the steps which comprise treating the acids prior to the introduction thereof in the apparatus with sulphurous acid produced by adding a sulphite thereto and maintaining the materials in the apparatus out of contact with oxidizing substances.

10. In a process of protecting metallic apparatus constructed of metal, the base of which is copper, against corrosion by acetic acid normally containing a cupric salt, the

steps which comprise treating said acid with a reducing agent prior to the introduction thereof in the apparatus and maintaining the acid in the apparatus out of contact with oxidizing substances. a 11. In a process of protecting metallic apinthe apparatus out of contact with oXidizing substances. 1

12. A process of protecting 'metallic apparatus, the base of which is copper, against corrosion by carboxylic acid introduced therein which comprises sweeping out the empty apparatus with an'inert gas, removing the molecular oxygen and the oxygencontaining compounds which are capable of corroding the apparatus, from said acid prior to the introduction thereof in the apparatus and protecting the materials within the apparatus from oxidizing influences.

13. A process of protecting metallic apparatus, the base of which is copper, against corrosion by acetic acid normally containing a cupric salt which comprises sweeping out the empty apparatus with an inert gas, passing an inert gas through said acid to remove" the molecular oxygen, treating the acetic acid with a reducing agent to remove the cupric salt, introducing said treated acid into the apparatus and protecting the materials within the apparatus from oxidizing influences.

In testimony whereof, I have affixed my signature to this specification.

NICOLAS BENOIT GRILLET'. 

